1. Field of the Invention
The present invention relates to fluorescent glass having a fluorescent property, an optical amplifier waveguide made of the fluorescent glass, and an optical amplifier module including the optical amplifier waveguide.
2. Related Background Art
Optical amplifier modules used in optical communication systems and the like compensate for the loss incurred in signal light during when the signal light propagates through optical transmission lines, and can optically amplify the signal light in optical amplifier fibers by supplying the optical amplification fibers with pumping light. For example, an optical amplifier module including an optical amplifier fiber doped with Er element (EDFA: Erbium-Doped Fiber Amplifier) can optically amplify signal light in C band (1530 nm to 1565 nm) or L band (1565 nm to 1625 nm) by using pumping light in the wavelength band of 0.98 μm or 1.48 μm. An optical amplifier module including an optical amplifier fiber doped with Tm element (TDFA: Thulium-Doped Fiber Amplifier) can optically amplify signal light in S band (1460 nm to 1530 nm) by using pumping light in the wavelength band of 0.8 μm, 1.05 μm, 1.2 μm, 1.4 μm, or 1.55 to 1.65 μm.
Fluorescent glass comprising a composition made of Al2O3 (3.7 wt %), SiO2 (94.0 wt %), and Bi2O3 (2.2 wt %) has been disclosed in Yasushi Fujimoto et al., “New Emission Characteristics of Bi-Doped Silica Glass in the 1.3-μm Band”, IEICE Trans. C., vol. J83-C, No. 4, pp. 354–355 (April 2000). The fluorescent glass comprising the composition containing Bi2O3 has a fluorescence spectrum over a wide band with a fluorescence peak wavelength near a wavelength of 1.24 μm. On the other hand, no absorption spectrum has been seen in this fluorescent glass within the wavelength range of 900 nm to 1600 nm. Therefore, an optical amplifier module constituted by an optical amplifier fiber made of this fluorescent glass is expected to optically amplify signal light in a band including a wavelength of 1.3 μm which is a zero-dispersion wavelength of typical single-mode optical fibers.
However, the fluorescence peak of the fluorescent glass disclosed in Non-patent Document 1 mentioned above exists near the wavelength of 1.24 μm shifted on the shorter wavelength side from the wavelength of 1.3 μm, whereby the efficiency of power conversion from pumping light to signal light in optical amplifier modules using optical amplifier fibers made of this fluorescent glass does not always seem to be favorable.
In the case of EDFA, its gain spectrum at the time of actual use is obtained when the absorption spectrum is subtracted from the fluorescence spectrum according to population inversion. Therefore, the gain peak wavelength does not always coincide with the fluorescence peak wavelength of 1.53 μm, but may rather exist near a wavelength of 1.55 μm to 1.56 μm on the longer wavelength side of the fluorescence peak wavelength. In the case of EDFA, the gain spectrum form is determined by population inversion, whereby a flatness can be realized in the gain spectrum when the population inversion is adjusted to 40% by regulating respective powers of pumping light and input signal light, for example. By contrast, the fluorescent glass disclosed in Non-patent Document 1 does not exhibit absorption but fluorescence, whereby the form of gain spectrum is considered to be identical to the form of fluorescence spectrum. Therefore, optical amplifier modules using optical amplifier fibers made of this fluorescent glass do not seem to attain a gain spectrum with a favorable flatness no matter how the respective powers of pumping light and signal light and the product of concentration and length of optical amplifier fibers are adjusted.
The fluorescent glass disclosed in Non-patent Document 1 has such a high melting point that it is made by heating to a temperature of 1760° C. Therefore, its manufacturing equipment becomes larger in scale, and the making is not always easy.
For overcoming the problems mentioned above, it is an object of the present invention to provide fluorescent glass, an optical amplifier waveguide, and an optical amplifier module which can realize a flat gain spectrum and are easy to make.